A rotary furnace hearth has a rotary hearth comprised of a hearth carriage having a propulsion system and on which hearth refractories are placed. The raw material is carried on the top surface of the hearth refractories and heated from above for sintering and reduction. This rotary hearth is constructed from a plurality of hearth carriages coupled in a ring from the viewpoint of fabrication and transport. As the coupling structure, a welded structure 50 welding members 14a and 14b of adjoining hearth carriages shown in FIG. 10 or a multi-stage bolted structure 52 using two rows of bolts 53 above and below shown in FIG. 11(a) has generally been used to make the hearth as a whole an integral structure.
However, the hearth carriages of this rotary furnace hearth receive heat from the inside of the high temperature furnace resulting in the temperature of the top surfaces becoming higher than the temperature of the bottom surfaces. As a result, if making the hearth carriages 10 an integral structure, as shown in FIG. 12(a), there was the problem that the difference in heat expansion caused the inner circumference side wheels 32 of the hearth carriages 10 to rise and excessive weight to concentrate at the outer circumference side wheels 33 and obstruct propulsion. Furthermore, there was the problem that the rise of the inner circumference side wheels 32, as shown in FIG. 13, caused interference between the hearth refractories and the wall refractories.
To avoid these problems, a structure enabling the hearth carriages 10 to freely move so as to give the state after heat deformation shown in FIG. 12(b) becomes necessary. The conventionally employed multi-stage bolt coupling structure 52 shown in FIG. 11(a) gives freedom to the coupling between hearth carriages by gradually loosening the nuts 54 in accordance with the heat deformation of the hearth carriages 10 and thereby enables heat deformation to be absorbed as shown in FIG. 11(b). However, when the furnace cools, the bolts and nuts loosen and therefore rattling unavoidably occurs between the hearth carriages 10. Therefore, there was the defect that while cold, the hearth cannot be kept circular and rotation is obstructed. There was the problem that to prevent this, it is necessary to adjust the amount of tightening of the bolts and nuts with each temperature change such as heating or cooling due to operation.